In frequency-division duplexing (FDD) radio communication transceiver systems like UTRAN or CDMA-2000, as well as in full-duplexing systems, the radio transmitter (Tx) of the transceiver system can be active at the same time as the radio receiver (Rx). The Rx and the Tx can use separate dedicated frequency bands, as in FDD, or can share the same band, as in full-duplexing systems. The separation between the Rx and the Tx can be provided, for instance, by antenna filtering, RF filtering, baseband filtering, and/or the like.
In such systems, a portion of the transmitted signal may be captured by the receiver of the same transceiver. In some cases, this can be due to the transceiver having a single Tx/Rx antenna, using a sharp filter, or the like, to separate Tx from Rx. The “transmit leakage” from a Tx branch to a Rx branch can cause, for instance, the Tx signal to overload the Rx circuit and bring the Rx front-end into compression or even damage it. Moreover, Rx sensitivity can be degraded by the Tx noise in the Rx band. Consequently, it may be desirable to attenuate wide-band noise from the PA on the receiving channel to a sufficient level that the Tx noise contribution becomes small compared to the thermal noise on the Rx input.
Recent developments in radio frequency (RF) front-ends for handheld devices, for instance, include more flexible and tunable filtering. Spatial duplexing and active filters are offered for full duplex and FDD systems to isolate the Tx from the Rx. Generally, active filtering is performed by creating a cancelation path to the leakage path from the Tx to the Rx. For best results, the cancellation path has the same amplitude and opposite phase to the Tx leakage signal. However, because of the proximity effect of the user on the handheld devices and handset antennas, as well as other environmental factors, determining factory settings for such types of filters is problematic, since the Tx leakage signal can be dynamically changing.